1. Field of the Invention
The present invention relates to image forming apparatuses and methods like those employed in the form of copying machines, printers, facsimile devices, and the like. More particularly, the invention relates to a method and apparatus capable of eliminating dirt of a recording paper's rear surface and preventing an insufficient cleaning of the transfer medium.
2. Description of the Related Arts
In an image forming apparatus utilizing an electrophotographic process, the electrostatic latent image formed on the image carrier such as photosensitive drum, etc. is developed with toner or the like. The toner image thus formed thereon is transferred onto the recording medium (hereinafter, called "recording paper") such as transfer paper, overhead projection (OHP) film, etc. As to the transfer medium, there exist two types: a non-contact type transfer medium represented by a corona charger, and the like; and a contact type transfer medium represented by a transfer belt, a transfer roller, etc. In recent years, there is an increasing trend to employ the non-contact type transfer medium, primarily because of its superior electric current efficiency, image quality, and apparatus maintenance ease.
As presently observed, in a developing apparatus used to apply toner (or other image forming substance) to an electrophotographic latent image formed on an image carrier, a portion of the toner is charged with a polarity opposite to that of the normal charge imparted on the toner. The occurrence of such inversely charged toner tends to happen very often in the initial state of shipping the product from the factory or in the initial state of exchanging the developer. Another cause of a portion of the toner becoming inversely charged is due to an aging of the toner and a deterioration of carrier that mixes with the toner.
When the inversely charged toner attaches to a non-image portion (that is, a background portion) on the image carrier, this portion of toner attaches to the transfer medium physically (not electrostatically, but mechanically) by pressing against the image carrier when employing the contact type transfer medium.
The attachment phenomenon is especially pronounced when the transfer medium remains in contact with the image carrier when performing successive copying operations. Under these conditions, the inversely charged toner existing on the non-image portion of the image carrier corresponding to the area between the adjacent recording papers becomes attached to the transfer medium on some occasions. As a result, if the transfer process for a next recording paper is performed while leaving the inversely charged toner attached to the transfer medium, the inversely charged toner attaches to the rear surface of the recording paper and results in dirtying the rear surface of the recording paper.
The published specification of Japanese Laid-open Patent Publication No. 8-110712/1996 discloses one approach for preventing the rear surface of the recording paper from becoming dirty due to the inversely charged toner. The technology of the image forming apparatus described in the above specification takes a countermeasure of preventing the inversely charged toner from being transferred onto a transfer belt by applying a transfer bias to the non-image portion (on the image carrier) corresponding to the area between the adjacent recording papers when performing a successive copying operation.
However, although the image forming apparatus as described in the above specification can prevent the transferring of the inversely charged toner by applying the transfer bias to the non-image portion (the area between the adjacent papers), an undesirable phenomenon of normal toner (the normally charged toner) becoming electrostatically absorbed onto the transfer medium occurs inevitably. Since the normally charged toner attached to the transfer medium is electrostatically absorbed, a sufficient cleaning operation cannot be performed by way of a transfer belt cleaning member, etc. and thereby a phenomenon of "toner filming" tends to occur very often.
As previously mentioned, the attaching of the inversely charged toner to the transfer medium and the attaching of the normally charged toner thereto are very closely related to each other. The countermeasure of coping with both of the problems to be solved requires mutually contradictory treatments. Furthermore, it is impossible to effectively prevent both of the inversely charged toner and the normally charged toner from being attached to the transfer medium in the conventional image forming apparatus. Those are the problems to be solved in the prior art, as recognized by the present inventors.